The invention relates to a method for encoding data using a differential space-time block code (DSTBC), to a method for decoding data encoded and transmitted in this way, and to a transmitting device and receiving device respectively for carrying out such methods.
A DSTBC scheme for two transmit antennas was described in V. Tarokh and H. Jafarkhani, “A differential detection scheme for transmit diversity,” IEEE Journal on Selected Areas in Communications, vol. 18, pp. 1169-1174, July 2000. DSTBC is based on STBC (space-time block code), described in S. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE Journal on Selected Areas of Communications, Special Issue on Signal Processing for Wireless Communications, vol. 16, no. 8, pp. 1451-1458, 1998. Another description of DSTBC may be found in C. Gao, A. Haimovich, D. Lao “Bit Error Probability for Space-Time Block Code with Coherent and Differential Detection”, IEEE Vehicle Technology Conference (VTC '02 Fall), September 2002, Vancouver, Canada. A generalization of DSTBC is given in H. Jafarkhani and V. Tarokh, “Multiple transmit antenna differential detection from generalized orthogonal designs,” IEEE Transactions on Information Theory, vol. 47, pp. 2626-2631, September 2001.
Similarly to STBC, an individual DSTBC codeword or an individual DSTBC code occupies two time slots of the transmission carrier, each individual code carrying two information symbols. For DSTBC technology, no channel information is required, either at the receiver end or at the transmitter end, which makes this technology very attractive, especially in cases where reliable channel information is difficult to obtain. With DSTBC technology, a differentially encoded code sequence Ck is recursively calculated according toCk=Sk·Ck-1.  (1)
The next code matrix Ck in each case is calculated by multiplying a current information matrix Sk by the previous code matrix Ck-1. This means that the information matrix Sk is differentially encoded similarly to differential phase modulation schemes. The information matrix Sk is generated on the basis of two information symbols s1,k and s2,k which must be transmitted according to an information matrix structure with
                              S          k                ⁡                  (                                                                      s                                      1                    ,                    k                                                                                                s                                      2                    ,                    k                                                                                                                        -                                      s                                          2                      ,                      k                                        *                                                                                                s                                      1                    ,                    k                                    *                                                              )                                    (        2        )            
If the information symbols s1,k and s2,k are taken from a pure PSK (phase shift keying) constellation diagram, the absolute values are normalized according to
                                                    s                          1              ,              k                                                =                                                        s                              2                ,                k                                                          =                                    1                              2                                      .                                              (        3        )            
In this case it follows from (2) thatSk·Sk*==I2.  (4)
This means that for modulation schemes of increasingly higher order compared to QAM (quadrature amplitude modulation), PSK degradation becomes increasingly greater, virtually reaching 10 dB for modulation schemes with 6 bit/s/Hz bandwidth efficiency.
The performance of differential single-antenna transmission techniques and higher-level DPSK (differential PSK) modulation is described in H. Rohling and V. Engels, “Differential amplitude phase shift keying, (DAPSK)—a new modulation method for DVBT” in International Broadcasting Convention, pp. 102-108, 1995. DAPSK modulation schemes are found to be much more efficient than pure PSK technology. This analysis was the motivation for a similar design of differential modulation schemes for DSTBC which simultaneously uses amplitude and phase shift keying (APSK). Proposals for integrating APSK into DSTBC are described in X.- G. Xia, “Differentially en/decoded orthogonal space-time block codes with APSK signals” IEEE Communications Letters, vol. 6, pp. 150-152, April 2002, G. Bauch, “A Bandwidth-efficient Scheme for Non-coherent Transmit Diversity”, in Proc. of IEEE Globecom Conference, San Francisco, Calif., USA, Dec. 1-5, 2003 and G. Bauch, “Differential Amplitude and Unitary Space-Time Modulation”, in Proc. of 5th International ITG Conference on Source and Channel Coding, Erlangen, Jan. 14-16, 2004, the general approach being based on introducing differential amplitude modulation over the entire sequence of code matrices Ck to be transmitted. This technique is analytically described byCk·Ck*=ak·I2,  (5)which results in different amplitudes of the code blocks transmitted. The actual information is coded into the amplitude ratio
      a    k        a          k      -      1      via two mutually adjacent matrices Ck and Ck-1. In this case, demodulation is performed separately for amplitude and phase bit.
The basis is therefore DSTBC for two transmit antennas as one of various MIMO (multiple input multiple output) transmission methods. Similarly to a coherent space-time block code in accordance with STBC, DSTBC is based on a complex orthogonal design. However, DSTBC requires no channel information. In the case of original DSTBC, only one pure PSK modulation with correspondingly very limited bandwidth efficiency was used. As an enhancement, an amplitude difference between two consecutive DSTBC blocks has also been encoded, the two information symbols within one such individual DSTBC block always having the same amplitude. However, the envelope of the transmission signal is not constant.
The article Hwang, C.- S. “Differential Space Time Block Codes Using Nonconstant Modulus Constellations”, IEEE Transactions on Signal Processing, Vol. 51, No. 11, November 2003, pp. 2955-2964, likewise discloses information encoding by differential space-time block codes.